The physical feature for ferroelectric crystal is the spontaneous polarization (Ps), and the ferroelectric domain to reverse its spontaneous polarization (Ps). It offers an alternative 180° inversion, i.e., so-called domain inversion, and the capability to apply an electric field across the polar axis to overcome the coercive field (Ec)in ferroelectric. One such example takes advantage of the reversible polarization and fast switching time to realize high-density memory devices for data storage, such as: S. Essaian “Nonvolatile memory based on metal-ferroelectric-metal-insulator semiconductor structure” U.S. Pat. No. 5,899,977 and O. Auciello et al. “The physics of ferroelectric memories”, Physics Today, pp. 22-27, July, 1998.
In ferroelectric nonlinear crystals, the 180° change in the spontaneous polarization (Ps) offer optics nonlinear coefficient of odd-order physical tensor by sign change. Based on the following of: N. Bloembergen, U.S. Pat. No. 3,384,433 and “Interactions between light waves in a nonlinear dielectric”, Phys. Rev. vol. 127, pp. 1918-1939, 1962, offer a wave front vector K=2π/Λ approach by compensating effect of dispersion for the coefficient of refraction to overcome a phase-matching problem (ktω−2kω≠0) when frequency transformation and satisfy the use of a Quasi-Phase-Matching (QPM) technique as shown in FIG. 1 with the relatively physical structure to satisfy ktω−2kω≠0 and length of per inverted domain with Λ=2lc at lc=λ/4(n2ω−nω).
As mentioned above, M. Yamada et al. U.S. Pat. No. 5,193,023, “Method of controlling the domain of a nonlinear ferroelectric optics substrate”, and “The first-order quasi-phase-matched LiNbO3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generation,” in Appl. Phys. Lett. Vol. 62, pp. 435-436, 1993, that discloses a short pulse voltage is applied QPM structure of periodically poled lithium niobate (PPLN) for second-harmonic-generation (SHG) green laser. Y. Kitaoka et al., “Miniaturized blue laser using second harmonic generation”, in Jpn. J. Appl. Phys. Vol. 39, pp. 3416-3418, 2000, that discloses the tiny QPM-SHG blue-laser technique such as 5×12×1.5 mm3. Using the 20 mW-infrared semiconductor laser as the pumping source, the power of blue laser transformation is 2 mW.
The summary of technique problems for applying to QPM inverted domain structure comprises the item of:                (1). the situation becomes complicated on fabricating of small periodical inverted-domain structure due to the existence domain merge in the polarization switching process.        (2). improved the separation of fundamental frequency and frequency transformation light such as the same polarization direction and parallel each other to propagate the light.        
As discussed herein, the first problem reason belong to fringing field effect due to a large dielectric discontinuity underneath the electrode pattern. As shown in FIG. 2(A) of a conventional poling configuration of z-directional poling diverted voltage for normal field (Ez) enhanced current spreading in the unpatterned regime, which is normally coated with a layer of insulating material. The distribution of switching current and charged field play an important role in determining the arrangement of the reversed domains. Therefore, an external supply of the switching current, i.e., 2AdPs/dt serves to compensate the depolarization field in the newly switching area A. According to analysis of static electricity as shown in FIG. 2(B) of a conventional poling configuration of z-directional poling diverted voltage, a relatively large non-isotropy Ex˜4Ec can exist underneath the insulating layer. Ex is its field line pointing to an inward direction from both sides underneath the insulating layer for unnecessary inverted domian. This phenomenon shows to result in field screening and compensating in the unpatterned regime to bring up the issue of domain broadinging, and deviates the QPM condition formed on its lattice structure for low efficiency of nonlinear optical transformation.
In addition, the reason of problem (2) is due to the QPM condition limit wave frequecy transformation to the wave collinear incidence for in FIG. 1. Because the wave front of the structure with periodical diverted domain parallel the said wave collinear incidence, the fundamental wave, wave of transformaton, and wave front of the structure with periodical diverted domain paralleling each other can not distingu distinguish one from the others. Therefore, the conventional QPM device must take another filter to divide into the fundamental wave and wave of transformaton that caused great inconvenience.
Other the conventional problems are proposed as follows:    (1). The insulating layer in FIG. 3, it is to restrain the domain necleation and motion in a subsequent field poling process for fringing field of ferroelectric nonlinear crystals covered by periodical metal eletrode for inverted domain.
First, the SiO2 material is applied at the isolating layer as shown in K. Mizuuchi et al., “Generation of ultraviolet light by frequency doubling of a red laser diode in a first-order periodically poled bulk LiTaO3”, Appl. Phys. Let. Vol. 70, pp. 1201-1203, 1997.
Next, the Ta2O5, WO3, HfO3 material is applied at the isulating layer as shown in M. C. Gupta et al., “Method of inverting ferroelectric domains by application of controlled electric field”, U.S. Pat. No, 5,756,263.
Subsequently, the spin-on-glass material is applied at the insulating layer as shown in G. D. Miller et al., “42%-efficient single-pass cw second-harmonic generation in periodically poled LiNbO3”, Opt. Lett. Vol. 22, pp. 1834-1836. 1997.
As mentioned above, the Appl. Phys. Lett. Vol. 70, pp. 1201-1203, 1997 issue can be related to fringing field enchanced current spreading (Wmin) in the unpatterned regime, which is normally coated with a layer of insulating material such as SiO2, the experienced form between Wmin and substrate thickness (T) is Wmin=0.0027T−0.21 (μm).
Then, the Wmin=0.0027T−0.21 (μm) is subjected to through said substrate thickness (T) to decide to fabricate the accurate method of the least period. For example, the 500-μm-thick LiNbO3 structure carry out together with the application of said substrate thickness (T) and the least error of the inverted domain erred from the path of metal eletrode pattern is 1.14 μm. In addition, the 850-nm-thick QPM-SHG structure can be fulfilled by periodical domain diversion at coherent length (lc) lc=1.9 μm, then the substrate require lapping and polishing to become thin for the allowable error of QPM period.
Matsushita Electric Industrial Co., Phys. Lett. Vol. 70, pp. 1201-1203 (1997)], that discloses the substrate such as LiTaO3 require lapping and polishing to 150-μm thickness for fabricating the inverted domain with a 1.7 μm period in QPM structure.
Sony Corp., Yamaguchi et al., “Method of local domain control on nonlinear optical materials”, U.S. Pat. No. 5,526,173, that discloses the substrate such as LiNbO3 require lapping and polishing to 100-μm thickness.
On the other hand, the substrate such as LiTaO3 and LiNbO3 having hard coefficient above 5 scale can be dirctly stressed to the irregural diverted domain of end surface of ferroelectric nonlinear crystals using stress-induced piezoelectricity because of the substrate require lapping and polishing to become thin. As a result the pulsed field poling is inevitably controlled problem of periodcal inverted domain and yielding rate.    (2). The diffusion technique for controlling the inverted domain is to during the high-temperature treatment or chemical treatment using the lithium-ion difussion of the inner crystals because of local inverted domain. Generally, the conventional techniques are proposed as follow: (a) titanium (Ti) diffusion, (b) proton exchange, (c) high-temperature lithium-ion out-difussion, (d) the oxide covered and heat treatment, such the (a) and (d) take place shallow surface domain inversion in the uncovered eletrode pattern regime and (b) take place shallow surface domain inversion in the uncovered metal pattern regime. The diffusion technique can support the big-area inverted domain using shallow surface domain inversion because of the temperature vs. time being exponential saturation. As the result the difussion technique is inevitably induced non-perpendicular boundary of inverted domain and changeable strauture and physical property.
S. Miyazawa, “Ferroelectric domain inversion in Ti-diffused LiNbO3 optical waveguide,” J. Appl. Phys. Vol. 50, 4599-4603, 1979, that discloses the titanium (Ti)-diffusion technique.
M. L. Bortz et al., “Noncritical quasi-phase-matched second harmonic generation in an annealed proton-exchange LiNbO3 waveguide”, IEEE Quantum Electron. Vol. 30, pp. 2953-2960, 1994, and K. Nakamura et al., “Antipolarity domain nucleation and growth during heat treatment of proton-exchanged LiTaO3”, J. Appl. Phys. Vol. 73, p. 1390, 1993, that discloses the proton-exchange technique.
K. Nakamura et al., “Ferroelectric domain inversion caused in LiNbO3 plates by heat treatment”, Appl. Phys. Lett. Vol. 50, pp. 1413-1414, 1987, that discloses the high-temperature lithium-ion out-difussion technique.
M. Fujimura et al., “Ferroelectric domain inversion induced by SiO2 cladding for LiNbO3 waveguide”, Elec. Lett. Vol. 27, pp. 1207-1209, 1991, that disclosed the oxide covered such as SiO2 and heat treatment.
C.-S. Lau et al., “Fabrication of MgO induced lithium out-diffusion waveguides,” IEEE Photon. Technol. Lett. Vol. 4, pp. 872-875, 1992, that disclosed the oxide covered such as MgO and heat treatment.
As diffusion technique problems are proposed as follows:                1. The difussion only with shallow surface domain inversion take place in a triangular pattern in the uncovered LiNbO3 regime, and in a half-circle pattern in the uncovered LiTaO3 regime, the inverted domain can not give the the idea perpendicular boundary that as shown in FIG. 1 and can give an unwanted influence to the nonlinear wave transformation.        
K. Yamamoto et al., “Characteristics of periodically domain-inverted LiNbO3 and LiTaO3 waveguides for second harmonic generation”, J. Appl. Phys. Vol. 70, pp. 1947-1951, 1991, that discloses the boundary of periodical inverted domain in contrast to transformation efficiency.                2. The proton exchange cause the lattice composition and structure to form LixH1-xNbO3 and decrease the coefficient of nonlinear optical transformation. The resultant substrate is subjected to heat treatment of annealing for decreasing effect of the mentioned above, but said coefficient of nonlinear optical transformation restores the limited degrees. Y. N. Korkishko et al., “The SHG-response of different phase in proton-exchanged lithium niobate waveguide”, IEEE J. Selected Topics in Quantum Electron. Vol. 6, pp. 132-142,2000, that discloses the proton exchange of the mentioned above.            (3). the decreasing coercive field (Ec) of the lump lattice to restrain the fringing field underneath the eletrode pattern. It can not change the optical nonlinear characteristic and transformation efficiency of the lattice, because of said decreasing coercive field (Ec) of the lump lattice reduces the defective density of lattices.
The conventional methods are proposed as follows:                1. Stoichiometric nonlinear crystals offers composition of Li2O/(Nb2O5+Li2O) or Li2O/(Ta2O5+Li2O) for 0.49−0.5 approach by substantially decreasing the coercive field to 2 kV/mm that is one tenth of congruent-grown crystals. V. Gopalan et al., “Lithium niobate single crystal and photo-functional device,” U.S. Pat. No. 6,195,196 and V. Gopalan et al., “Lithium tantalate single crystal and photo-functional device,” U.S. Pat. No. 6,211,999, that discloses the said stoichiometric nonlinear crystals.        2. MgO or ZnO doping in congruent-grown crystals using the the material of atomic bond equivalent to LiNbO3 or LiTaO3 such as A. Harada, “Fabrication of ferroelectric domain reversal”, U.S. Pat. No. 5,568,308, that discloses the MgO doping in congruent-grown crystals, and L,-H. Peng et al., “Method for bulk periodic poling of congruent grown ferroelectric nonlinear optical crystals by low electric field”, U.S. Pat. No. 6,295,159, that discloses the ZnO doping in congruent-grown crystals.        
When doping in congruent-grown lithium niobate (LiNbO3) is Li2O/(Nb2O5+Li2O)=0.485 wherein the doping control into the consistency range of 3˜9 mol % for reducing lattice defects, the coercive field can effectively reduce one tenth of congruent-grown crystals and carry out the resistant-light destructiveness and resistant-light deflection.
As mentioned above, when doping in congruent-grown crystals for decreasing the coercive field, it must control the doping accuricy to overcome the induced law lattice quality for over doping and without compensated lattice defects for under doping.    (4). The pulsewave controlling offers the adjustment the field wave and duty cycle between the positive and negative electrode to shift the inverted domain such as: K. Mizuuchi et al., “Method for manufacturing domain-inverted region, optical wavelength conversion device utilizing such domain-inverted region and method for fabricating such device”, U.S. Pat. No. 5,652,674 and R. G. Batchko et al., “Back-switching poling in lithium niobate for high-fidelity domain patterning and efficient blue light generation”, Applied Physics Letters, Vol. 75, pp. 1673-1775. Said the pulse-wave controlling must appropriate liquid eletrode in order to avoid the dielectric collapse of material. Such as: R. L. Byer et al., “Electrical field domain patterning,” U.S. Pat. No. 5,800,767 and U.S. Pat. No. 6,156,255.